This invention concerns a current regulator circuit for a load that is controlled by a power driver stage, for example, for a DC motor that is controlled by pulse-width modulation or by timing pulses.
In some current regulator circuits such as those used for servo motors, a power driver stage comprises four transistors connected as a bridge with the motor being connected between the bridge diagonals. In such circuits the current that flows in the two bridge diagonals is measured, and the actual analog voltages generated by such measurements are conducted directly to a comparison point in a control stage, with the comparison point being DC-connected to the power driver stage. An error signal is generated upon comparison of the actual analog voltage with a desired or theoretical value, which is likewise present in analog form. The error signal is then amplified by a control amplifier with PI (proportional-integral) time behavior to produce an output voltage that controls the duty cycle of the bridge diagonals.
With prior art systems a disadvantage of the analog current regulator circuit is its low bandwidth which is on the order of 1 kHz. The circuit responds so slowly to changes in the load current as a consequence, for example, of overshoot processes or interference that for safety reasons the current must be limited to a value that is considerably less (e.g., 30% less) than the peak current that is intrinsically allowable. Even more important problems arise if one wants to connect transistors that are suitable for high voltages directly to the existing AC main without the intermediate connection of expensive power transformers. Such transistors are presently available for power driver stages, and their direct connection can be effected through a rectifier arrangement. In this case, however, one must take care to provide DC isolation of the power driver stage from the control stage including its power supply and from all the remaining electronic control mechanisms of the system, which frequently includes a process computer or other data processing system. The necessary DC isolation makes it difficult to reach a high transmission bandwidth, a good linearity, and the desired accuracy and resolution of the regulator circuit. It is particularly difficult to transmit the actual-value signal, corresponding to the load current, into the control stage with the desired bandwidth. Further problems arise if essential elements of the control circuit must be situated in the isolated power driver stage where they require a separate current supply system.